The present invention relates to an X-ray CT apparatus and an image reconstructing method which, when continuously measuring an approximately same cross section, are suitable for obtaining at high speed a cross sectional (CT) image the artifact of which is reduced.
The X-ray CT apparatus has been widely used, and are being utilized in a variety of ways by users. In recent years, especially when endermicly performing an organization inspection of a lesion or a treatment therefor, the CT is getting more and more employed as a guide to a centesis. Performing such an operation under the guide by the CT, which makes it possible to reduce the time needed for the operation and at the same time to improve the accuracy thereof, is being considered to be a helpful and promising method.
For such a continuous observation of the CT image, it becomes necessary to reconstruct the image at high speed. Concerning such a high-speed image reconstructing method, several proposals have been made up to the present time.
In JP-B-1-23136, a second image having a time difference is obtained by obtaining difference amount between projection data used to reconstruct a first image and projection data needed to obtain a second image, and then adding a difference amount image, which is obtained by reconstructing the difference amount data, to the first image.
Disclosed in JP-A-8-24252 is a method in which a new cross sectional image, after reconstructing a first image, is obtained using the first image and a new image obtained by reconstructing projection data ((1/N) view: N indicates the number of division) which is insufficient to reconstruct a complete image. Disclosed in JP-A-8-24253 is a method similar to this.
All of the above-described methods, by partially reconstructing an image, make it possible to update the image at high speed. An ordinary scanning is a 360.degree./scan, and one cross sectional image is reconstructed from 360.degree. amount of data. On the other hand, the technique disclosed in the JP-A-8-24252 makes it possible to obtain a completed sectional image by obtaining an image (i.e. a partial reconstructed image) by reconstructing projection data for every, for example, 30.degree. and then adding necessary number (in the case of 30.degree., 12 images) of the partial reconstructed images.
FIG. 5A is a diagram showing a manner of parallel beam projection data in two views (Illustrated in the Figure are two views at 0.degree. and 30.degree.). Here, parallel projection datalization means that a fan beam X-ray is converted into a parallel beam in terms of the mathematical expressions. FIG. 5B is a diagram showing an embodiment of memory storing of projection data P (P.sub.1, P.sub.2, . . . ) divided in a unit of 30 .degree. as indicated by 0.degree. to 30.degree., 30.degree. to 60.degree., . . . .
FIG. 5C is a diagram showing an embodiment of memory storing of a partial reconstructed image g (g.sub.1, g.sub.2, . . . ) and a normal reconstructed image r (r.sub.1, r.sub.2, . . . ).
Here, the partial reconstructed image g means each of reconstructed images obtained from the parallel projection data in the width of 30.degree.. The partial reconstructed image g1 means a reconstructed image obtained from the parallel projection data in a range of 0.degree. to 30.degree., and the partial reconstructed image g.sub.2 means a reconstructed image obtained from the parallel projection data in a range of 30.degree. to 60.degree., and the other partial reconstructed images mean the same.
The normal reconstructed image r designates a 360.degree. amount of reconstructed image obtained by composing (usually, adding) the plurality of partial reconstructed images g. Namely, the normal reconstructed image r means an ordinary CT image. The partial reconstructed image g which is obtained from the parallel projection data in the width of 30.degree. does not constitute a complete image owing to the lack of data. Accordingly, the normal reconstructed image is obtained by adding the twelve partial reconstructed images g obtained in the width of 30.degree.. Moreover, the addition of the partial reconstructed images in the width of 30.degree. is performed sequentially. Such a sequential addition brings about an advantage of enabling the normal reconstructed images to be obtained one after another in real time. Incidentally, it is a well-known matter that the reconstruction can be executed through the addition.
Namely, in the FIG. 5C, a first normal reconstructed image r.sub.1 is obtained by adding the partial reconstructed images from g.sub.1 to g.sub.12. A second normal reconstructed image r.sub.2 is obtained by adding the partial reconstructed images from g.sub.2 to g.sub.13. The images from g.sub.2 to g.sub.12, which are needed for the second normal reconstructed image r.sub.2, have been already determined when the first normal reconstructed image r.sub.1 is obtained. Thus, what is newly required is only the image g.sub.13. The image g.sub.13 is obtained through a scanning in a range of 0.degree. to 30.degree. out of a next scanning in a range of 0.degree. to 360.degree., without waiting for a finish of the next whole 360.degree. amount of scanning. Consequently, the time needed to obtain the data becomes 1/12th as compared with the one needed for the single whole scanning, and the normal image r.sub.2 could be obtained by just adding the 30.degree. amount of partial reconstructed image.
In this way, the second image r.sub.2, if the thirteenth partial reconstructed image g.sub.13 is further reconstructed, is obtained by adding the thirteenth image to an added partial reconstructed image obtained by adding the images from g.sub.2 to g.sub.12. Thus, it becomes possible to obtain a new cross sectional image in a time of summation of a time .DELTA.t.sub.1 for reconstructing the one partial reconstructed image g.sub.13 and a time .DELTA.t.sub.2 for adding thereof, and in addition the time for the reconstruction is proportional to the number of back projection views. This makes it possible to reconstruct the one partial reconstructed image with a lapse of time about 1/12th as compared with the time needed for the ordinary reconstruction, thus allowing a high-speed image reconstruction. If the time, i.e. .DELTA.t.sub.1 +.DELTA.t.sub.2, is equal to 1/12th of the scanning time, it is possible to obtain the cross sectional images continuously in real time in parallel with the measurement. Concerning the third or after of normal reconstructed images r.sub.3, r.sub.4, too, the above-described description is given in much the same way.
A basic concept in any of these algorithms is increasing a speed. Meanwhile, an object to be scanned is, in many cases, moving in a real time CT scanning. The example is a monitoring of a centesis needle or a measured cross section at the time of the spiral scanning. In the above-described prior arts, however, no consideration was given to a suppression of the artifact which is a characteristic of the CT apparatus and appears when such a moving object is imaged.
Also, the artifact in the CT apparatus appears most outstandingly from a discontinuity in the data between the times of starting and finishing of the measurement. This is the point that the people concerned appreciate very well. For example, when reconstructing the image with the use of 180.degree. amount of data, the problem is a discontinuity between the data at 0.degree. and the one at 180.degree. which are in an opposite relationship to each other in terms of the projection direction. Although correction algorithms for such a discontinuity have been already used, it is difficult to execute them in view of the operation time thereof. Accordingly, in the process for the real time reconstructed image, it was usual to omit the correction algorithms.